Page Items * linear information * a three step process * crossing over * location * counting events Biological information is linear Biological information is stored on DNA molecules in the form of long string of "letters"; the sequence of nitrogenous bases adenine (A), guanine (G), thymine (T), and cytosine (C). Show These "letters" spell out three letter "words" that specify single amino acids in a polypeptide chain, and it is the appropriate sequence of these three letter code words, or codons, that specifies the exact order or sequence of the amino acids in a polypeptide. A gene is a linear sequence of codons that, together with regulatory sequences (at the beginning and sometimes the end) and sometimes "junk" DNA inserts, specifies either a complete protein or a subunit of a protein. Genes and other pieces of regulatory DNA are in a linear sequence along large pieces of DNA that are complexed with other proteins into chromosomes. All biological information is somehow arranged in a linear manner with a specific order or sequence to the component parts. Of course, the complete picture of how organisms code for, store, find, express and eventually use this information is much more complex than this simple picture, but at its core all biological information is stored as a linear sequence of larger and larger units. A linear sequence of bases become codons, a sequence of codons becomes a gene, and a sequence of genes becomes a chromosome. Page Items * linear information * a three step process * crossing over * location * counting events Since the time of Mendel, biologists have tried to find out just where in the library of information a specific piece of information is stored. For a long time, this involved a three step process;
Today, human technology has progressed to the point that scientists can now determine the exact sequence of all the bases along all the DNA molecules in genomes as complex as our own, but that is a very recent skill. To some extent, however, even this "hi-tech" approach to dissecting biological information still depends on older technologies of gene mapping. The problem, then and now, was how to "map" a gene on a chromosome. How do you determine where a gene is located? How do you determine how far along the chromosome one gene is from the next? Page Items * linear information * a three step process * crossing over * location * counting events Crossing over For a very long time the answers to both these questions could be found in following what happened to genes in a diploid cell as that cell undergoes meiosis. In diploid cells, homologous chromosomes carry the same genes, in the same sequence along their length. When two sets of these chromosomes pair in meiosis I they frequently exchange parts, a phenomenon known as crossing over. [Note:this may not be completely the case all the time, but it is generally true for most chromosomes and genes] As far as we know, the chances of crossing over at any one point on a chromosome is a completely random event. So, if two genes are close together there will be a lower chance of crossing over between them than if the two genes were very far apart on the same chromosome. Page Items * linear information * a three step process * crossing over * location * counting events Location For example: chromosome #1---URA-----TRY-------------------- chromosome #2---ura-------try-------------------- The two genes "URA" and TRY" are close together on this homologous chromosome pair (#1 and #2), so there is little chance of a crossing over event occuring between them. In meiosis I, therefore, these two genes will stay on the same chromosome and travel together into the same haploid gamete. Genes stay together, the closer they are to each other on the same chromosome. However: chromosome #1---URA----------------------TRY--- chromosome #2---ura-------------------------try--- In this example, the "URA" and "TRY" genes are much further apart on the same chromosome. At meiosis I, therefore, there is a much higher chance of a crossing over event between them. They will usually separate with their new chromosomes into different haploid gametes. If the two genes separate out into different gametes, they are much further apart on chromosome. Page Items * linear information * a three step process * crossing over * location * counting events Counting crossing over events Any technique that measures the frequency with which crossing over occurs between two genes on the same chromosome can thus measure the distance these two genes are apart. For example, the following data can give us a "map" of the relative locations of these three genes; URA, TRY and PEP.
In this organism: There is only one way this data can be interpreted: ----URA-------14-------TRY----9----PEP----- The sequence of the genes it URA-TRY-PEP and the distances between them are "14" units and "9" units respectively. Genes can be positioned on chromosomes by finding out how many times a pair of genes separates into the same haploid in meiosis I and how many times they separate into different haploid products at meiosis I. Science@a Distance © 2002, Professor John Blamire How are genes arranged on a chromosome?Genes are arranged linearly along the length of each chromosome (like beads on a string), with each gene having its own unique position or locus. In a pair of chromosomes, one chromosome is always inherited from the mother and one from the father.
What is a genes location along a chromosome called?Locus. Locus is a term that we use to tell us where on a chromosome a specific gene is. So it's really the physical location of a gene on a chromosome. It's a way of defining the gene's neighborhood.
Where are genes arranged?Genes are contained in chromosomes, which are in the cell nucleus. A chromosome contains hundreds to thousands of genes. Every normal human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes.
Does genes have specific locations on chromosomes?The genes on each chromosome are arranged in a particular sequence, and each gene has a particular location on the chromosome (called its locus). In addition to DNA, chromosomes contain other chemical components that influence gene function.
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Genes are arranged in specific locations and in a specific order along __________ chromosomes.
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